(1) Field of the Invention
Multilayer printed circuit boards typically made of epoxy glass resin with copper innerlayers having drilled thru-holes are well known. The present invention relates to preparing these drilled holes for the application of the conductive metal, usually electrolessly plated copper, and more specifically, relates to chemically treating the surface of the thru-holes to increase its effective surface area to provide improved coverage of the metal deposited on the surface.
(2) Description of the Prior Art
Thru-holes in printed circuit boards must have adequate and uniform copper coverage to provide a good electrically conductive surface. A blistered, voided or flaked appearance on the plated hole wall shows lack of metal coverage of the copper on the substrate. Several steps are necessary to prepare drilled thru-holes for reception of the electroless metal.
During the drilling of thru-holes in a printed circuit board comprising epoxy resin and copper multilayers, resin can be smeared over the copper innerlayers, and, if the holes are left untreated prior to copper deposition on the hole well, a dielectric barrier will be left between the conductive path of applied copper plate and copper conductor layers. The dielectric barrier interferes with the circuitry.
It is important to remove the epoxy resin smear before metallizing the holes. The prior art provides techniques for removing smear including mechanical means such as vapor honing which abrades or blasts the smear away or a secondary drilling operation with a slightly larger drill.
Chemical processes are the most commonly used methods for removing smear. Concentrated sulfuric acid (H.sub. 2SO.sub. 4) or chromic acid solutions are most commonly used to dissolve the epoxy resin smear. Plasma etching in vacuum chambers is also a technique which can be used for removing smear. British Pat. No. 1,479,556 discloses a process using alkaline potassium permanganate. Chemical means besides removing smear, can also provide an etchback by removing more of the epoxy resin from the hole walls thereby exposing the sides of the copper innerlayer in addition to the edge of the innerlayer, which etchback process some believe provides a more reliable plated thru-hole. FIG. 1 shows a cross section of a portion of an etched back hole showing the copper plate with resin on either side, the surface of which has been electrolessly copper plated.
When concentrated sulfuric acid is used to desmear and etchback the hole surface, a precipitate of dissolved resin may be formed on the surface. This is thought to happen because as the concentrated sulfuric acid is diluted by rinsing, it cannot maintain in solution as much resin which then precipitates out. This precipitate is known as etchback residue and must be removed prior to plating. Water rinses are usually not sufficient to do this. Aqueous solvents for the residue have been developed which effectively remove this residue and leave a clean surface on the hole wall as shown in FIG. 4.
Subsequent to the removal of etchback residue, conventional glass etchants may be used to remove protruding glass fibers prior to electroless plating. The protruding glass fibers without this etching may cause misplating or result in uneven metal coverage. Glass etching improves the ability of the glass to be plated and provides more even metal coverage.